<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>媒体查询</title>
    <style>
        *{
            padding:0px;
            margin:0px;
            box-sizing: border-box;
        }
        body{
            padding: 20px;
        }
        .header,.left,.center,.right{
            border:1px solid black;
            margin-top:5px;
            text-align: center;
        }
        .left,.center,.right{
            height: 300px;
            line-height: 300px;
        }
        .header{
            width: 100%;
            height: 50px;
            line-height:50px;
        }
        @media screen and (min-width:992px){
        .left,.right,.center{
            float:left;
        }
        .left{
            width: 30%;
        }
        .right{
            width: 30%;
        }
        .center{
            width:40% ;
        }
        }
        @media screen and (min-width:768px) and(min-width:991px){
        .left,.center,.right{
            float:left ;
        }
        .center{
            width: 60%;
        }
        .left{
            width: 40%;
        }
        .right{
            width: 100%;
        }
}
      @media screen and (max-width:767px){
          .left,.center,.right{
              float:left;
              width:100%；
          }
      }

    </style>
</head>
<body>
    <header class="header">header</header>
    <aside class="left">left</aside>
    <section class="center">center</section>
    <section class="right">right</section>
</body>
</html>
